Volumetric measuring method and apparatus

ABSTRACT

Ullage volume of a container containing a fluid is determined by injecting a known quantity of radioactive material into the ullage volume. After the radioactive material has dispersed uniformly throughout the ullage volume the density thereof is measured to provide the ullage volume indication. The system can be utilized to monitor the amount of fuel in the container under either zero G or a gravitational field. The ullage volume indication can be compensated for pressure variations within the volume by utilizing a pressure transducer having an output that is combined with the detected density. A system is provided to compensate for background, radiation noise. The system can also be utilized to detect leakage from the container by noting changes in concentration of the radioactive material. Corrections for temperature variations are also provided. In fixed volumes systems, the device can be utilized to monitor pressure and temperature variations.

United States Patent Joyce [111 3,745,338 July 10, 1973 VOLUMETRICMEASURING METHOD AND APPARATUS [75] inventor: William B. Joyce,Columbus, Ohio [73] Assignee: Industrial Nucleonics Corporation {22]Filed: Aug. 17, 1964 [21] Appl. No.: 390,002

[52] U.S. Cl. 250/435 FL, 73/l49, 250/106 T [51] Int. Cl. G0ln 23/12[58] Field of Search 250/435 D, 43.5 FL, 250/435 PG, 43.5 R, 106 IL, l06T, 83.6 FT;

[56] References Cited OTHER PUBLICATIONS Apparatus for Measuring HumanBody Volume, by W.

E. Siri, from Rev. of Scientific Instruments, Vol. 27, No. 9, Sept.1956, pgs. 729738.

Primary.Examiner-Archie R. Borchelt Att0rneyWilliam T. Fryer, Ill, JamesJ. OReilly and Cushman, Darby & Cushman [57] ABSTRACT Ullage volume of acontainer containing a fluid is determined by injecting a known quantityof radioactive material into the ullage volume. After the radioactivematerial has dispersed uniformly throughout the ullage volume thedensity thereof is measured to provide the ullage volume indication. Thesystem can be utilized to monitor the amount of fuel in the containerunder either zero G or a gravitational field. The ullage volumeindication can be compensated for pressure variations within the volumeby utilizing a pressure transducer having an output that is combinedwith the detected density. A system is provided to compensate forbackground, radiation noise. The system can also be utilized to detectleakage from the container by noting changes in concentration of theradioactive material. Corrections for temperature variations are alsoprovided; In fixed volumes systems, the device can be utilized tomonitor pressure and temperature variations.

43 Claims, 10 Drawing Figures 2 42 //7 D a ,[COUNT gggqy 3O .36 f- RATEI j 34 r KR as j j SUPPLY 38 4O 2 2 1 v Z 29 .i'- m- IT --7--.. i "-i"in? T b l 26 ENGINES VOLUMETRIC MEASURING METHOD AND APPARATUS Thisinvention relates generally to volumetric gauging apparatus and moreparticularly to a novel method and means for determining the capacity ofan irregular volume by means of an identifiable tracer.

Many times it is required to measure the total volume defined by a rigidbody. One example is found in subterranean surveying where the volume ofa salt-lined cavern must be known if it is to store natural gas. Anotherexample exists where the total interior volume of ceramic evaporatorplates must be determined. While in these cases the volume is irregular,it is fixed by the rigid walls of the surrounding medium. Suppose,however, in the first example after the cavern is surveyed, that it ispartially filled with a liquid such as oil. Then the volume of the spaceabove the oil will be a measure of how much oil remains. This unfilledspace is commonly referred to as ullage. When the total volume of acontainer is known, a ullage measurement yields useful fill information.

Several methods have been tried to solve the volumetric gauging problem.Sometimes a gas is pumped into the cavity and its volumetric flow rateis integrated until a sharp increase in pressure is noted. Thisindicates that the unknown volume is completely occupied by the fillinggas and the unknown volume is equal to the volume of gas pumped into thecavity. Other methods rely on the phenomenon of acoustical resonance.

Since the resonant frequency of a cavity is dependent upon the volumethereof, sound energy of that frequency coupled into the cavity will bereinforced while those waves of a different frequency will not.Generally, the volume will be inversely proportional to frequency.Another method is described in a June 1952 ASTIA report ATI 166,433entitled Investigation of Fuel Quantity Measuring Technique. Here, inthe vapor above fuel in a tank, a small reference volume is establishedinto which the ullage can flow. A microphone is mounted in thisreference volume and another is placed in the main ullage to determinethe ullage volume, the standard volume is isolated from the ullage and apair of pistons cooperate to compress the vapor in each volume by aknown incremental amount. At the end of compression, the pressure ineach volume is measured by the microphone sensors. The fuel volume isproportional to the difference in the respective pres-' sures.Primarily, since these methods rely on mechanical interactions with themeasurand, most are extremely inaccurate and awkward to use in mostenvironments.

In accordance with my invention, I release into the unknown volume aknown amount of.an identifiable tracer such as a radioactive gas thatwill disperse uniformly throughout the region to be measured. I thenmeasure the concentration of the tracer, i.e., the amount of tracer perunit volume. In this respect concentration may be considered synonymouswith density. Since the. mass of the material diffused is known, itsdensity will be inversely proportional to the unknown volume into whichit spread. Moreover, the measurement will be insensitive to substantialpressure changes in the volumesresident gas since the transducer willrespond principally to the radioactive tracer or probe gas.

In a specific embodiment, my invention finds particular utility as aliquid propellant gauge for rocket propulsion systems such as used inspace vehicles designed for sub-orbital, orbital or interplanetarytravel. In these systems, it is common to expel the fuel with apressurant gas acting either directly against the surface of thepropellant or against a diaphragm. In some cases, the vapor pressure ofthe liquid propellant itself is sufficient to expel the fuel. Therefore,if no diaphragm is used the ullage includes both propellant vapor andperhaps pressurant gases. When the system enters a zero-g environment,the propellant becomes weightless and may assume a variety of surfaceconfigurations. Some of the ullage may be trapped in the fuel andeffectively isolated from the above-described prior art sensing devices.My gauging method is not dependent on the fuelullage interfaceconfiguration. During pre-launch, or later, I inject a known number ofcuries of a radioactive gas such as krypton 85 into the ullage spaceabove the fuel and mount a radiation detector in communication with theullage. The maximum distance that radiation will travel before it isabsorbed is inversely proportional to the density of the resident gas.My detector responds only to the radiation of those atoms located lessthan this distance away and thus, in effect, measures the number ofcuries in a known sample volume of the ullage. Alternatively, a samplingvolume can be mechanically established by substantially enclosing thedetector in shielding that prevents radiation external to the mea suringvolume from entering the detector. I further provide means forcorrecting the detector output in the event the probe gas is partiallysoluble in the fuel or lost through a leak in the system. Correctionsare also made for temperature and pressure variations that may affectthe system under certain conditions.

Accordingly, it is a primary object of the present invention to providea novel method for measuring the interior of either a regularly or anirregularly shaped 7 container.

It is another object of the present invention to provide a volume gaugefor tankages that is independent of the surface configuration assumed bythe stored product.

It is also an object'of the present invention to provide a ullagemeasuring system that is insensitive to a large degree to changes in thepressure of the resident gas.

It is a further object of the present invention to provide a volumegauge that does not depend on moving mechanical parts subject to wear.

It is still another object of the present invention to provide a volumegauge that is simpler and more compact in construction than similardevices used heretofore. i

. It is yet another object of the present invention .to provide a volumegauge that is more accurate and reliable than similar devices currentlyin use.

These objects as well as other features of the present invention willbecome more apparent upon reference to the following descriptionwhentaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view illustrating the volumetric. gaugingmethod of the present invention;

FIG. 2 is a graph illustrating the response of the detector used in themethod shown in FIG. 1;

FIG. 3 is a sectional view partly diagrammatic of a fuel gauging systemusing a radioactive tracer;

FIG. 4 is a graph of count rate vs time for the radiation measuringsystem shown in FIG. 3;

FIG. 5 is a more detailed schematic view of a shieldless detectorillustrating the dependence of detector range upon pressure;

FIG. 6 is a sectional view of a shielded detector;

FiG. 7 is a sectional view of an improved shielded detector;

FIG. 8 is a block diagram of a complete fuel gauging system constructedin accordance with the present invention;

FIG. 9 is an alternative embodiment of the present invention formeasuring pressure; and

FIG. 10 is another embodiment comprising a temperature gauging system.

With reference now to the drawings and particularly to FIG. 1, theirregularly shaped interior volume of a body 10 is measured inaccordance with the present invention by releasing into the interior 11from a cell 12 a known amount of a probe gas such as hydrogen, chlorineor other identifiable substance. The probe gas eventually will diffusethroughout the interior by intermingling with whatever gases residedthere before the release of the probe gas. In the simple diffusion ofone gas into another, the concentration of either component obeys thefollowing partial differential equation:v

where C is the concentration of the gas component, I is time, x, y and zare position coordinates and D is the diffusion coefficient. After asufficient length of time has passed, the concentration gradients vanishand diffusion is complete.

Since a known amount of gas in has spread throughout an unknown volume,the concentration C of the probe gas will be inversely proportional tothe unknown volume V according to flnal A detector 14 communicating withthe interior 1 1 is responsive only to the concentration of the probegas. Detector 14 can be any of the commercially available halogendetectors or other detector responsive to the probe gas being used. Itshould be apparent, however, that a probe gas not already present orresiding in the interior volume should be selected; otherwise, m is notknown precisely. Alternatively, the interior can be exhausted of allresident gases before injection of the probe gas. It should be apparentthat the known mass m must occupy the unknown volume V. Error may resultif the walls absorb some of the test gas.

FIG. 2 graphically shows the response of a typical concentrationdetector. The slope of characteristic 16 is representative of thesensitivity of the measuring method and can be made quite large bycareful choice of probe gas and detector type. One particular selectionI have found desirable is a beta-emitter such as krypton 85. Otherradioisotopes of different energy and type of radiation may be moresuitable in certain lowtemperature gauging environments; however, thisradioisotope, in relatively small quantities, when measured by aninexpensive radiation detector, yields an extremely accurate volumegauge. This combination is described in the following embodiment of thepresent invention wherein its utility will become immediately evident.

The measurement of an irregular volume is intimately related to themeasurement of fuel in a tank of a space vehicle subjected to periods ofweightlessness or zero gravity that cause the fuel to randomly orientitself in the tank. A typical fuel tankage is illustrated in FIG. 3. Atank 20 contains a quantity of fuel 22 that is pumped at 24 through anoutlet conduit 26 to the rocket engines. Into the ullage space 28 abovethe fuel is injected a known mass of krypton through an inlet 30. Theradioisotope will diffuse uniformly through the tank ullage by mixingwith the fuel vapor. The amount dissolving into the propellant itselfwill normally be insignificant. In other systems, a pressurant gas mayalso be present in the ullage to force the fuel out of the tank 20. Itmay be desirable to provide a blower 31 to hasten the diffusion of theradioisotope. Tritium, a radioisotope of hydrogen, may be used insteadof krypton 85, especially in the measurement of very low temperatureliquid fuel tankages.

As the fuel supply decreases, the ullage volume increases. Since theamount of radioisotope is fixed at a certain number or fraction of acurie, the curie density of the ullage must necessarily diminish. Curiedensity is defined for the purpose of this disclosure as a count rateper unit volume. This is related, of course, to the number ofradioactive atoms per unit volume. The ullage curie density isindependent of the fuel-vapor interface shape. For example, neither theparaboloidal interface shape 29 nor the trapped ullage bubble 31 alterthe curie density, of the ullage, provided there is no entrapped ullageat the time of injection. The fuelvapor interface would assume the shape29 whenever there is a simultaneous thrust and rotation about the longaxis of the tank 20. To sample the ullage, a radioactive detector 32 ismounted inside the tank to measure the radioactivity in a small knownvolume 34 of ullage outlined by the dotted line 36. Alternatively, thedetector could be mounted outside the tank if the ullage mixture couldstill be sampled. It is important, however, that the fuel 22 be kept outof the sampling volume because it will significantly reduce the amountof detected radiation due to its relatively high radiation absorptioncapability. Construction will be described hereinafter that effectivelyeliminates this problem. A conventional count rate circuit 38 isconnected to the detector 32 to provide a signal on line 40 proportionalto the counts per second, N, generated in the detector 32 by theradioactivity in the sample volume 34. The count rate N can be expressedas follows:

N km,/V,

where m, is the mass of radioactive tracer in the sampling volume V andk is a constant of proportionality. Each curie of tracer in the samplevolume will provide 3.7 X 10 disintegrations per second. Other radiationapparatus such as an ion current device would operate in my inventionwith equal utility.

Referring briefly to FIG. 4 the count rate N may experience a transientrise after injection at time T,,. Shortly, the count rate levels off atN, which is indicative of a full tank. Of course, some ullage space mustalways exist in the vicinity of the detector. At launch time T fuel iswithdrawn from the tank and the count rate decreases. A meter 42connected to the count rate circuit 38 could be used to register thecount rate. Ac-

cordingly, the face of meter 42 may be calibrated directly in units offuel volume.

The system will be accurate despite any complicated fuel-vapor interfaceconfiguration that may develop during zero-g or normal flight maneuversbecause the curie density of the ullage is not dependent on what shapethe fuel is in. The total ullage volume remains constant and changesonly when fuel is withdrawn or where there is a leak. A leak may bedetected by observing the count rate when no fuel is being used. If aleak is detected, it will be necessary to fix it and establish anotherreference for the tank for changes in the remaining fuel. It may bedesirable to recharge the ullage anyway to increase the detector signalat low fuel levels.

I also provide compensation for adverse changes in the temperature andthe pressure of the system that might be reflected in the count rate.For example, a decrease in thetemperature increases the solubility ofthe probe gas thereby decreasing the observed count rate. This problemdoes not obtain, of course, in those tanks using a diaphragm, i.e.,bladder. Similarly, an increase in pressure increases the amount ofabsorber between the source and the detector, thereby decreasing thecount rate N. If the fuel is assumed to be of negligible solubility overthe expected range of temperatures, only the problem of highpressureremains. In a practical situation, the reference volume would bemade sufficiently small so that for the range of pressure normallyencountered, negligible absorption occurs in the resident gas. If asensitivity problem would arise because of the small sampling volume,either multiple sample volumes or a more penetrating radioactive gascould be used.

According to the ideal gas law, pressure P, volume v, and temperature Tare related by Pv nRT where n is the number of moles of gas in thevolume v and R is a gas constant. My gauge measures the density n/v ofthe radioactive tracer. If either temperature or pressure is fixed orknown, the other is uniquely determined. Some functional relationshipother than the one stated by equation (4) may apply in a non-ideal gassituation. Besides affecting the solubility of the tracer gas in thefuel, temperature variations cause dimensional changes in the tank anddensity changes in the propellant being used. Correction devices toeliminate temperature-excited errors are described hereinafter.

FIG. 5 shows schematically how an unshielded detector may be responsiveto pressure variations. Assuming a pressure P radiation from anyparticular atom a will travel, on the average, a distance L before it isfor all practical purposes absorbed by the vapor. If the detector 32 hasa viewing angle of 180, it will respond to those atoms located within ahemispherical volume of radius L. The sampling volume-v may be definedby the following equation:

However, if the pressure should drop to the value P while the density ofthe radioactive atoms was held constant, the effective range of theradiation is in creased. Atoms not counted before are now sampled. Themeter 42 will register a higher count rate than would be representativeof a higher gas concentration. But, in fact, the concentration did notchange, since there is substantially the same number of radioactiveatoms per unit volume as before the pressure change. If necessary tocorrect the fuel gauge for pressure varia tions, a pressure transducer46 could be used to monitor the ullage pressure and generate anelectrical signal on line 48 that will correct the counting circuitoutput signal on line 50. The meter 42 will then be substantiallyinsensitive to pressure changes occurring in the ullage and willaccurately indicate the concentration of radioactive atoms in theullage.

Alternatively, a mechanical fabrication to rigidly define the samplingvolume can be constructed as shown in FIG. 6. In this embodiment, theface of detector 32 communicates with the interior of an open tophousing 54 having a top plate member 56. The walls of the housing 54 andthe plate member define a rectilinear volume into which the radioactiveullage] may flow as indicated by the streamlines 58. If the radiationpath length is much greater than the depth L, of the sampling volume,the detector responds to all the radiation originating in the samplingvolume. At very high pressures, radiation from beyond the dotted line 60may not make it to the detector 32, and the volume sampled iscorrespondingly smaller. This being the case, a smaller curie densitywill be measured and unless a pressure correction is made an erroneousindication of ullage volume will be made. Therefore, a pressure sensingcorrection system must be utilized whenever the ullage pressure becomesso high that the fixed sampling volume'measurement of the ullageprovided by the housing 54 is circumvented. It is apparent, however,that the shielded detector will be insensitive to pressure variationsbelow a certain maximum value determined in accordance with themagnitudeof the sampling volume.

Attached to either side of thedetector housing 54 may be a pair ofsource dispersal units 62, one being a spare. Each unit includes afrangible capsule 64 containing the radioactive substance to bedispersed. A pin 66 driven by a solenoid 68 may be used'to rupture thecapsule 64 and effect the aforesaid diffusion of radioactive substancein the ullage. The entire source-detector assembly may be mounteddirectly in the tank wall or otherwise coupled into the interior ofthetank.

Operation in outer space may pose several problems. one may be the highradiation levels in certain regions or belts that tend to mask theresponse of the measur ing detector 32 in background noise." Toeliminate this problem the output of a second detector 73 responsive tobackground radiation only and an associated counting circuit 75 issubtracted from the ullage count. The difference may be indicated onmeter 42. This technique of eliminating background radiation is thesubject of a copending application Ser. No. 148,594, filed Oct. 30,I96], and assigned to the same assignee as the present invention.

It may be a problem keeping the fuel itself from contaminating thedetector. To keep the fuel out of the detector and yet permit freecirculation of the ullage vapors therearound, the apparatus shown inFIG. 7 may be constructed. Here the detector 32 is mounted within acylindrical member 70 located in between a plurality of verticallystacked wafers 72 each having a centrally located hole 74. The wafersare spaced from one another to allow the ullage gas to flow into thesampling volume more or less defined by the aligned holes 74 and a pairof solid end plates 76 and 78. It is apparent that better ullagecirculation is provided by this construction. If the entire assemblyshould be submerged in a cohesive fuel in the absence of gravity, noneshould penetrate the sampling volume, since, in order to do so, a largearea of fuel would necessarily have to spread out and a somewhat largersystem energy than a minimum surface energy would prevail. The tendency,of course, is for the fuel to assume that configuration that providesthe minimum surface energy. Should droplets of fuel enter the samplinGvolume, a blast of compressed air would serve to clear them from thesampling volume. Vibratory techniques may be equally useful. Shutterdevices that temporarily close off the sampling volume may also beemployed.

It is evident from the above that factors other than fuel expendituremay effect a decrease in curie density. For example, some of theradioactive ullage, M may be lost through venting of the tank, anotherportion, M may dissolve into the fuel itself and an amount M may leakthrough holes caused either by micrometeorites or faulty tankconstruction practices. If M, is the total mass of radioactive substanceinjected originally into the ullage, the amount M in the ullage duringflight may be expressed as follows:

Referring now to FIG. 8, the amount M can be measured by detecting theradiation leaving the vent 80 by means of a detector 82. The amount Mcan be detected by either a detector submerged in the fuel or a detector84 monitoring what radioactivity is lost with fuel forced out of theoutput pipe 86 by a pump 88. In either case, the total mass ofradioactive substance lost will be a product of the density, thecross-sectional area, and the volumetric flow rate of the respectivelymonitored flows integrated in time. The venting flow rate may be assumedto be constant but the fuel flow rate may be quite variablebutmeasurable, for example, either bya tachometer 90 geared to a pumpdriving motor 92 or by taking the time derivative of the subject fuelgauge output'signal. A computer 93 receives information from the variousdetectors and sensors and provides a signal on line 94 that isproportional to the volume of fuel in the tank and corrected for all thevariables affecting the computation. Especially if there is for anyreason a drop in detector counting rate when no fuel is being used, acontrol unit 96 may be energized by the computer 93 to cause anindication on indicator 97, and actuation of one or both standby sourcedispersal units 98, 99. For example, if a leak should be detected in aseam 95 and repaired, an unknown amount of fuel would remain in thetank. After the repair, a count rate N is detected. The dispersal unit98 would release a quantity of gas that would produce by itself a countrate N The total count rate is Therefore, since N is known, N 'can' bededuced by subtracting N from N. This is just the same effect as if thenumber of curies were increased by a factor of (N N )/N over what itwould have been were the N count rate used alone. This provides aconstant correction to the computer 93 from the design calibration curveof the B source when expressed as a percent rather than in terms of anabsolute number of counts. What amounts to a change in measuring systemgain may be further useful on long missions in compensating for thehalf-life decay of the radioactive tracer.

The temperature of the system may also vary between wide extremes duringflight so a temperature sensor T can be located on the tank to provide acorrection signal to the measuring system. For example, correctionsshould be made for any thermally-excited changes in fuel tankdimensions. Since the density of the fuel is proportional totemperature, sensor T also may correct the computation of fuel mass,should such be desired. The system normally determines fuel volume bymeasuring the ullage volume but the total fuel mass is simply theproduct of the fuel density and the fuel volume. It is often moredesirable to know the total fuel mass remaining rather than the volumethereof. Accordingly, a mass computer 100 temperaturecorrected by thesignal on line 102 provides a total fuel mass signal and-line 104 themagnitude of which is indicated by the meter 106.

It will be apparent that while in a preferred embodi- 'ment of myinvention, I measure a changing volume and correct for temperature andpressure, that my method will measure either pressure or temperaturewhen one or the other is held constant in a system of fixed volume.

Referring briefly to FIGS. 9 and 10 in a fixed volume 108 located in anisothermal environment my invention will respond to the total pressurevariations of the resident gas. Because of the aforesaid absorption asthe pressure increases, the range of radiation becomes shorter and theresponse of curie density detector 109 drops off. To this end, a gauge110 and an indicator 112 may be used to develop the pressure-functionalreadout. Of course, a one-way valve ll3 or other filtering device isnecessary to prevent any radioactive gas from being lost. Conversely, inan isobaric environment, such as might be provided by a pressureregulator unit 114, a curie density detector 116, gauge 118 and anindicator 120 will provide a readout of temperature variationsoccasioned by any increase or loss of thermal energy Q as indicatedschematically by the large arrow 122. r

In most cases described above, the tracer disperses throughout theullage of a partially filled vessel. It may be desirable to use a tracerthat dissolves into and diffuses primarily through the fill material.Then a curie density measurement performed on the fill material willprovide a signal proportional to the volume of fill; however, a morepenetrating radiation should be used since there will be a much greaterattenuation in the fill than in the ullage.Moreover, this is a one-timeonly measurement, since the density will not change when fill is removedfrom the vessel.

While I have described my invention in terms of sev eral preferredembodiments, only those additions and modifications falling within thescope of the appended claims are within the spirit of my invention.

I claim:

1. The method of determining the volume of a region in a containercontaining an ullage region and a fluid occupying the remainder of thecontainer comprising the steps of:

injecting a known amount of radioactive material not otherwise presentinto said ullage region that disperses throughout said ullage region anddoes not substantially mix with the fluid, measuring with a detector forthe radioactive material the density of said injected material aftersaid material has uniformly dispersed throughout said ullage region andwhile the material is in the ullage region, and correlating saidmeasured density of said dispersed material with the volume of one ofsaid regions. 2. The method of determining the volume of a void regionin a container comprising the steps of:

injecting a known amount of an identifiable radioactive gas nototherwise present into said region that disperses throughout saidregion, measuring with a detector for the radioactive gas the density ofsaid injected gas in said void region after said gas has disperseduniformly throughout said region, and correlating said measured densityof said dispersed gas with the volume of said region. 3. The method ofdetermining the volume of a void region in a container comprising thesteps of:

injecting a known amount of radioactive material into said region thatdisperses throughout said region, measuring the density of said injectedradioactive material in said void region after said material hasdispersed uniformly throughout said region, and correlating saidmeasured density of said radioactive material with the volume of saidvoid region. 4. The method of determining the volume of a void region ina container comprising the steps of:

injecting a known amount of radioactive material into said void region,establishing a sampling volume of known magnitude in said void region,measuring the amount of said radioactive material in said samplingvolume, and correlating said mea sured amount of radioactive materialwith the volume of said void region.

5. The method of determining the volume of a void region in a containercomprising the steps of:

injecting a known amount of radioactive gas into said void region,

establishing a'sampling volume of known magnitude in said void region,

measuring the radiation emanating from said sampling volume, andcorrelating said measured radiation with the volume of said void region6. The method of determining the volume of a void region comprising thesteps of:

injecting a known number of curiesof radioactive material into said voidregion that disperses throughout said region,

measuring the curie density of said material in said void region afterit has dispersed throughout said void region, and correlating saidmeasured curie density of said dispersed material with the volume ofsaid void region.

7. The method of determining the volume of fuel remaining in a tank ofknown volume comprising the steps of:

injecting a known amount of radioactive gas substantially insoluble insaid fuel into said tank that disperses throughout said tank,

measuring the density of said injected gas in said tank after it hasdiffused throughout said tank, and correlating said measured density ofsaid diffused gas with the volume of said fuel.

8. The method of determining the volume of fuel remaining in a tank ofknown volume comprising the steps of:

injecting a known amount of radioactive gas insoluble in said fuel intosaid tank,

measuring the density of said radioactive gas after it has diffusedthroughout said tank to determine the ullage volume of said tank, and

comparing said ullage volume measurement together with said known totaltank volume to obtain said fuel volume.

9. The method of determining the volume of a region occupied by aresident fluid material comprising the steps of:

introducing a fixed amount of a radioactive probe 9 fluid into saidresident fluid to diffuse said probe fluid throughout said volume,measuring with a detector for the radioactive fluid the density of saiddiffused probe fluid in said region, and correlating said measured fluiddensity with the volume of said region.

10. The method of determining the volume of material contained in apartially filled vessel of known volume, comprising the steps of:

introducing a radioactive gas into the volume of said vessel notoccupied by said material that disperses throughout said non-occupiedvolume, measuring the density of said gas in said non occupied volumeafter it has diffused throughout said volume, and

correlating said measured density of said radioactive gas with thevolume of said material in said vessel. 11. The method of determiningthe volume of material contained in a partially filled vessel of knownvolume comprising the steps of:

introducing a radioactive test gas into the ullage volume of saidvessel, measuringthe density of said gas in said ullage vol ume toobtain a first signal having a first component proportional to saidullage volume and a second component proportional to the amount of gasdissolved in said material, measuring the amount of said gas dissolvedin said material, and combining said measurements in accordance with apredetermined relationship to derive an output sig nal proportional tothe volume of material in said vessel. 12. The method as set forth inclaim 11 but further including the steps of:

measuring the temperature and pressure of said ullage volume, and

correcting said output signal in accordance with said temperature andpressure measurement.

13. The method as set forth in claim 11 but further including the stepsof:

detecting leaks in said vessel,

stopping said leaks,

measuring the'density of said radioactive ullage after said leaks arestopped,

introducing a second test gas of known quantity into said ullage volume,and

measuring the density of said ullage after introduction of said secondtest gas. 14. The method of investigating a system having a resident gasfor changes in variable parameters such as volume, pressure ortemperature, comprising the steps of:

injecting a known amount of radioactive tracer into said gas thatdiffuses throughout said gas,

measuring the density of said injected tracer after said tracer hasuniformly diffused throughout said resident gas, and

correlating said measured density with one of said variable parameters.

15. The method of determining the pressure of a resident gas in a regionof fixed volume and temperature comprising the steps of:

injecting a radioactive tracer gas into said resident gas, said tracerdiffusing uniformly throughout said resident gas, and providingradiation having a range of penetration that is a function of saidresident gas pressure, and deriving a signal proportional to said rangeof radiation penetration for indicating the pressure of said residentgas. 16. The method of determining the temperature of a resident gas ina region of fixed volume and pressure comprising the steps of:

injecting a radioactive tracer gas into said resident gas,

said tracer diffusing uniformly throughout said resident gas andproviding radiation having a range of penetration that is a function ofsaid resident gas temperature, and

deriving a signal proportional to said range of radiation penetrationfor indicating the temperature of said resident gas.

17. The method of determining a leak in a normally closed systemcomprising the steps of:

injecting a known amount of radioactive tracer into said system todiffuse the same uniformly throughout,

measuring the density of said tracer in said system after diffusionwhile the system remains closed except for the leak, and

indicating any decreases in said measured tracer density.

18. Apparatus for determining the volume of a void region in a containercomprising:

means for releasing a known quantity of identifiable radioactivematerial into said volume,

said material being uniformly distributed throughout said volume, and

a detector for the radioactive material for measuring the density ofsaid uniformly distributed identifiable material in said void region.

19. Apparatus for determining the volume of a void region comprising:

means for dispersing a known amount of radioactive material throughoutsaid void region,

a radiation detector for measuring the density of said radioactivematerial dispersed in said void region, and

means for indicating said radioactive density measurement.

20. Apparatus for determining the volume of a void region comprising:

a fixed quantity of an identifiable radioactive tracer,

means for injecting said fixed quantity into said void volume touniformly disperse said tracer, and

detector means for the radioactive tracer responsive to the density ofsaid tracer in said void region for providing a signal that is afunction of said volume.

21. Apparatus for measuring the amount of material in a container ofknown volume comprising:

means for injecting a known quantity of a radioactive identifiablesubstance into the ullage volume of said container that dispersesthrough said ullage volume,

detector means for the radioactive tracer for measuring the density ofsaid identifiable substance in said ullage, and

means for correlating said density measurement with said material fill.

22. Apparatus for measuring the amount of fuel in a tank of known volumecomprising:

a radioactive tracer,

means for injecting a known amount of said tracer into the ullage ofsaid tank to establish a density that is a function of said fuel volume,

a radiation detector responsive to said tracer density and providing anoutput signal in accordance therewith, and

means for indicating said output signal.

23. Apparatus for measuring the amount of fuel in a tank of known volumecomprising:

a source of radioactive tracer emitting radiation,

means for injecting a known amount of said tracer into the ullage ofsaid tank to establish a uniform tracer density throughout said ullagevarying in ac cordance with the volume of said ullage,

a detector for measuring the radiation originating in a specific volumeof said ullage volume and generating an output signal in accordancetherewith, and

means for indicating said output signal.

24. Apparatus as set forth in claim 23 in which said specific volumedetector includes:

a generally hollow housing of shielding material mounted in said ullagevolume and having openings to permit said ullage to circulatetherethrough, and

a radiation sensor located inside said housing to receive radiationemitted only by the amount of said tracer located within the housing. A

25. Apparatus for measuring the amount of fuel in a tank of known volumecomprising:

a radioactive tracer emitting radiation,

means for injecting a known amount of said tracer into the ullage ofsaid tank to establish a uniform tracer density throughout said ullagevarying in accordance with the volume of said ullage,

a detector for measuring the amount of radiation in a known volume ofsaid ullage and generating a first signal that is a function of saidfuel volume and a function of the pressure of said ullage,

transducer means for providing a second signal that varies in accordance.with said ullage pressure, and

means for combining said signals to provide an output signal that is afunction only of said fuel volume.

26. Apparatus for measuring the amount of fuel in a 65 tank of knownvolume comprising:

a radioactive tracer emitting radiation, means for injecting a knownamount of said tracer into the ullage of said tank to establish auniform tracer density throughout said ullage varying in accordance withthe volume of said ullage,

a detector for measuring the amount of radiation in a known volume ofsaid ullage and generating a first signal proportional to said fuelvolume and the temperature and the pressure of said ullage,

temperature transducer means providing a second signal proportional tosaid ullage temperature,

pressure transducer means providing a third signal proportional to saidullage pressure, and

computer means for combining said first, second and third signals toprovide an output signal that is a function only of said fuel volume.

27. Apparatus as set forth in claim 26 in which said specific volumedetector comprises:

a generally hollow housing of shielding material mounted in said ullagevolume and having openings to permit said ullage to circulatetherethrough, and

a radiation sensor located inside said housing to receive radiationemitted only by the tracer occupying said housing.

28. Apparatus for measuring the amount of fuel in a tank of known volumecomprising:

a source of radioactive tracer emitting radiation,

means for injecting a known amount of said tracer into theullage of saidtank to establish a uniform tracer density throughout said ullagevarying in accordance with the volume of said ullage,

a detector for measuring the amount of radiation in a known volume ofsaid ullage and generating a first signal proportional to said fuelvolume and the temperature and the pressure of said ullage,

temperature transducer means providing a second signal proportional tosaid ullage temperature,

pressure transducer means providing a third signal proportional to saidullage pressure,

a second detector forgenerating a fourth signal pro- 1 portional to theamount of background radiation in the environment of said fuelmeasurement, and

computer means for combining said signals to pro vide an output signalproportional only to said fuel volume. 29. Apparatus for measuring theamount of fuel in a tank of known volume comprising:

a frangible capsule of compressed radioactive gas mounted in the ullageof said tank, means for rupturing said capsule dispersing a known numberof cuties of said gas uniformly throughout said ullage, v means formeasuring the curie density of said ullage including a detectorresponsive to the radiation of said radioactive gas, a housing forenclosing a fixed sampling volume in said ullage and means for mountingsaid detector in said housing, said detector providing a first signalproportional to said fuel volume, pressure and the temperature of saidullage volume, and the external radiation existing in the background ofsaid curie density measurement, pressure transducer means providing asecond signal proportional to said ullage pressure, temperaturetransducer providing a third signal proportional to said ullagetemperature,

a second detector for generating a fourth signal proportional to theintensity of said background radiation,

a first computer for combining said signals to provide a first outputsignal proportional only to said fuel volume, second computer meansreceiving said output signal and said third signal for providing asecond output signal proportional to the mass of said fuel in said tank,and means for registering an indication of said second output signal.30., Apparatus as set forth in claim 29 wherein said frangible sourcecomprises:

at least one beta-emitting isotope. 31. Apparatus as set forth in claim29 in which said sampling volume housing comprises: a center cylindricalshield member having a centrally located bore for mounting saiddetector, a plurality of washers each having a centrally located bore,means for mounting said washers in vertical align,- ment on either sideof said center cylindrical shield member and in alignment with thedetector mounting bore thereof, means for spacing said washers slightlyaway from one another, and end plate means for securing said mountingmeans and defining said specific volume with said aligned bores, 32.Apparatus for determining the pressure variations of a gas residing in afixed interior volume at constant temperature comprising:

means for injecting a radioactive gas into said resident gas,

said tracer diffusing uniformly throughout said resident gas andproviding radiation having a range of penetration that is a function ofsaid resident gas pressure, and

radiation detector means for providing an output sig nal proportional tosaid range of radiation penetration for indicating therpressure of saidresident gas. 33. Apparatus for determining the temperature of a gasresiding in a fixed interior volume at constant temperature comprising:

means for injecting a radioactive gas into said resident gas said tracerdiffusing uniformly throughout said resident gas and providing radiationhaving a range of penetration that is a function of said resident gastemperature, and

radiation detector means providing an output signal proportional to saidrange of radiation penetration for indicating the temperature of saidresident gas.

34. Apparatus for determining the volume of one fluid in the presence ofanother immiscible fluid in a container, said apparatus comprising: I

means for injecting into said container a known amount of radioactivesubstance that preferentially diffuses uniformly throughout only saidone fluid,

and

means coupled with said container for measuring the density of saiddiffused radioactive substance to provide a signal indicative of thevolume of said one fluid. 35. The method as set forth in claim 11 butfurther including the steps of:

introducing a second test gas of known quantity into said ullage volume,and measuring the combined density of said first and said second testgases.

36. In a volume measuring system, the combination of a container, afluid occupying a portion of the interior space in said container, meansconnected to said container for introducing a known mass of radioactivetrace gas into the ullage portion of said interior space and detectingmeans for the radioactive trace gas extending into said container formeasuring the density of said trace gas.

37. A method of determining the ullage volume of a partially filledcontainer having a changeable volume of material therein, comprising thesteps of:

introducing a radioactive gas into the volume of said vessel notoccupied by said material that disperses throughout said non-occupiedvolume,

measuring the density of said gas in said nonoccupied volume after ithas diffused throughout said volume, maintaining the mass of theradioactive gas substantially constant while the determination is beingperformed and the material volume is subject to change, and correlatingsaid measured density of said radioactive gas with the ullage volume insaid vessel. 38. A method of determining the ullage volume of apartially filled container having a changeable volume of materialtherein, comprising the steps of:

introducing a radioactive tracer gas into the volume of said vessel notoccupied by said material that disperses throughout said non-occupiedvolume,

measuring with a radioactive gas detector the density of said gas insaid non-occupied volume after it has diffused throughout said volume,

maintaining the mass of the tracer gas substantially constant while thedetermination is being performed and the material volume is subject tochange, and

correlating said measured density of said tracer gas with the ullagevolume in said vessel.

suring the density of said gas.

39. In a volume measuring system, the combination of a substantiallyclosed container, means for providing a mass of ullage gas to saidcontainer, means for introducing a known mass of radioactive trace gasinto said ullage gas and radioactive gas detecting means extending intosaid ullage gas for measuring the density of said trace gas.

40. In a system for measuring liquid or gas quantity, the combination ofa closed tank, containing a liquid or gas, whose quantity is to bemeasured, means for providing in said closed tank a mass of ullage gascontiguous with said liquid or gas, means for introducing a known massof radioactive trace gas into said ullage gas and radioactive gasdetecting means extending into said ullage gas for measuring the densityof said trace gas.

41. In a liquid quantity measuring system in a G or zero gravitationenvironment the combination of a closed tank containing a liquid whosequantity is to be measured, means for providing in said closed tank amass of ullage gas contiguous with said liquid, means for introducing aknown mass of radioactive trace gas into said ullage gas and radioactivegas detecting means extending into said ullage gas for measuring thedensity of said trace gas.

42. In a liquid quantity gauge for a zero gravitational environment thecombination of a closed tank having a diaphragm containing a liquidoccupying a portion of the interior space in said tank, means forproviding a known mass of trace gas into the ullage portion of saidinterior space and detecting means extending into said ullage space formeasuring the density of said gas.

43. In a liquid quantity gauge for a zero gravitational environment thecombination of a closed tank having a diaphragm containing a liquidoccupying a portion of the interior space of said tank, means forproviding in said closed tank an ullage gas to the ullage portion ofsaid interior space in said tank, means for providing a known mass oftrace gas into said ullage space and detecting means extending into saidullage space for mea-

1. The method of determining the volume of a region in a containercontaining an ullage region and a fluid occupying the remainder of thecontainer comprising the steps of: injecting a known amount ofradioactive material not otherwise present into said ullage region thatdisperses throughout said ullage region and does not substantially mixwith the fluid, measuring with a detector for the radioactive materialthe density of said injected material after said material has uniformlydispersed throughout said ullage region and while the material is in theullage region, and correlating said measured density of said dispersedmaterial with the volume of one of said regions.
 2. The method ofdetermining the volume of a void region in a container comprising thesteps of: injecting a known amount of an identifiable radioactive gasnot otherwise present into said region that disperses throughout saidregion, measuring with a detector for the radioactive gas the density ofsaid injected gas in said void region after said gas has disperseduniformly throughout said region, and correlating said measured densityof said dispersed gas with the volume of said region.
 3. The method ofdetermining the volume of a void region in a container comprising thesteps of: injecting a known amount of radioactive material into saidregion that disperses throughout said region, measuring the density ofsaid injected radioactive material in said void region after saidmaterial has dispersed uniformly throughout said region, and correlatingsaid measured density of said radioactive material with the volume ofsaid void region.
 4. The method of determining the volume of a voidregion in a container comprising the steps of: injecting a known amountof radioactive material into said void region, establishing a samplingvolume of known magnitude in said void region, measuring the amount ofsaid radioactive material in said sampling volume, and correlating saidmeasured amount of radioactive material with the volume of said voidregion.
 5. The method of determining the volume of a void region in acontainer comprising the steps of: injecting a known amount ofradioactive gas into said void region, establishing a sampling volume ofknown magnitude in said void region, measuring the radiation emanatingfrom said sampling volume, and correlating said measured radiation withthe volume of said void region
 6. The method of determining the volumeof a void region comprising the steps of: injecting a known number ofcuries of radioactive material into said void region that dispersesthroughout said region, measuring the curie density of said material insaid void region after it has dispersed throughout said void region, andcorrelating said measured curie density of said dispersed material withthe volume of said void region.
 7. The method of determining the volumeof fuel remaining in a tank of known volume comprising the steps of:injecting a known amount of radioactive gas substantially insoluble insaid fuel inTo said tank that disperses throughout said tank, measuringthe density of said injected gas in said tank after it has diffusedthroughout said tank, and correlating said measured density of saiddiffused gas with the volume of said fuel.
 8. The method of determiningthe volume of fuel remaining in a tank of known volume comprising thesteps of: injecting a known amount of radioactive gas insoluble in saidfuel into said tank, measuring the density of said radioactive gas afterit has diffused throughout said tank to determine the ullage volume ofsaid tank, and comparing said ullage volume measurement together withsaid known total tank volume to obtain said fuel volume.
 9. The methodof determining the volume of a region occupied by a resident fluidmaterial comprising the steps of: introducing a fixed amount of aradioactive probe fluid into said resident fluid to diffuse said probefluid throughout said volume, measuring with a detector for theradioactive fluid the density of said diffused probe fluid in saidregion, and correlating said measured fluid density with the volume ofsaid region.
 10. The method of determining the volume of materialcontained in a partially filled vessel of known volume, comprising thesteps of: introducing a radioactive gas into the volume of said vesselnot occupied by said material that disperses throughout saidnon-occupied volume, measuring the density of said gas in saidnon-occupied volume after it has diffused throughout said volume, andcorrelating said measured density of said radioactive gas with thevolume of said material in said vessel.
 11. The method of determiningthe volume of material contained in a partially filled vessel of knownvolume comprising the steps of: introducing a radioactive test gas intothe ullage volume of said vessel, measuring the density of said gas insaid ullage volume to obtain a first signal having a first componentproportional to said ullage volume and a second component proportionalto the amount of gas dissolved in said material, measuring the amount ofsaid gas dissolved in said material, and combining said measurements inaccordance with a predetermined relationship to derive an output signalproportional to the volume of material in said vessel.
 12. The method asset forth in claim 11 but further including the steps of: measuring thetemperature and pressure of said ullage volume, and correcting saidoutput signal in accordance with said temperature and pressuremeasurement.
 13. The method as set forth in claim 11 but furtherincluding the steps of: detecting leaks in said vessel, stopping saidleaks, measuring the density of said radioactive ullage after said leaksare stopped, introducing a second test gas of known quantity into saidullage volume, and measuring the density of said ullage afterintroduction of said second test gas.
 14. The method of investigating asystem having a resident gas for changes in variable parameters such asvolume, pressure or temperature, comprising the steps of: injecting aknown amount of radioactive tracer into said gas that diffusesthroughout said gas, measuring the density of said injected tracer aftersaid tracer has uniformly diffused throughout said resident gas, andcorrelating said measured density with one of said variable parameters.15. The method of determining the pressure of a resident gas in a regionof fixed volume and temperature comprising the steps of: injecting aradioactive tracer gas into said resident gas, said tracer diffusinguniformly throughout said resident gas, and providing radiation having arange of penetration that is a function of said resident gas pressure,and deriving a signal proportional to said range of radiationpenetration for indicating the pressure of said resident gas.
 16. Themethod of determining the temperature of a resident gas in a region offixed voLume and pressure comprising the steps of: injecting aradioactive tracer gas into said resident gas, said tracer diffusinguniformly throughout said resident gas and providing radiation having arange of penetration that is a function of said resident gastemperature, and deriving a signal proportional to said range ofradiation penetration for indicating the temperature of said residentgas.
 17. The method of determining a leak in a normally closed systemcomprising the steps of: injecting a known amount of radioactive tracerinto said system to diffuse the same uniformly throughout, measuring thedensity of said tracer in said system after diffusion while the systemremains closed except for the leak, and indicating any decreases in saidmeasured tracer density.
 18. Apparatus for determining the volume of avoid region in a container comprising: means for releasing a knownquantity of identifiable radioactive material into said volume, saidmaterial being uniformly distributed throughout said volume, and adetector for the radioactive material for measuring the density of saiduniformly distributed identifiable material in said void region. 19.Apparatus for determining the volume of a void region comprising: meansfor dispersing a known amount of radioactive material throughout saidvoid region, a radiation detector for measuring the density of saidradioactive material dispersed in said void region, and means forindicating said radioactive density measurement.
 20. Apparatus fordetermining the volume of a void region comprising: a fixed quantity ofan identifiable radioactive tracer, means for injecting said fixedquantity into said void volume to uniformly disperse said tracer, anddetector means for the radioactive tracer responsive to the density ofsaid tracer in said void region for providing a signal that is afunction of said volume.
 21. Apparatus for measuring the amount ofmaterial in a container of known volume comprising: means for injectinga known quantity of a radioactive identifiable substance into the ullagevolume of said container that disperses through said ullage volume,detector means for the radioactive tracer for measuring the density ofsaid identifiable substance in said ullage, and means for correlatingsaid density measurement with said material fill.
 22. Apparatus formeasuring the amount of fuel in a tank of known volume comprising: aradioactive tracer, means for injecting a known amount of said tracerinto the ullage of said tank to establish a density that is a functionof said fuel volume, a radiation detector responsive to said tracerdensity and providing an output signal in accordance therewith, andmeans for indicating said output signal.
 23. Apparatus for measuring theamount of fuel in a tank of known volume comprising: a source ofradioactive tracer emitting radiation, means for injecting a knownamount of said tracer into the ullage of said tank to establish auniform tracer density throughout said ullage varying in accordance withthe volume of said ullage, a detector for measuring the radiationoriginating in a specific volume of said ullage volume and generating anoutput signal in accordance therewith, and means for indicating saidoutput signal.
 24. Apparatus as set forth in claim 23 in which saidspecific volume detector includes: a generally hollow housing ofshielding material mounted in said ullage volume and having openings topermit said ullage to circulate therethrough, and a radiation sensorlocated inside said housing to receive radiation emitted only by theamount of said tracer located within the housing.
 25. Apparatus formeasuring the amount of fuel in a tank of known volume comprising: aradioactive tracer emitting radiation, means for injecting a knownamount of said tracer into the ullage of said tank to establish aunifOrm tracer density throughout said ullage varying in accordance withthe volume of said ullage, a detector for measuring the amount ofradiation in a known volume of said ullage and generating a first signalthat is a function of said fuel volume and a function of the pressure ofsaid ullage, transducer means for providing a second signal that variesin accordance with said ullage pressure, and means for combining saidsignals to provide an output signal that is a function only of said fuelvolume.
 26. Apparatus for measuring the amount of fuel in a tank ofknown volume comprising: a radioactive tracer emitting radiation, meansfor injecting a known amount of said tracer into the ullage of said tankto establish a uniform tracer density throughout said ullage varying inaccordance with the volume of said ullage, a detector for measuring theamount of radiation in a known volume of said ullage and generating afirst signal proportional to said fuel volume and the temperature andthe pressure of said ullage, temperature transducer means providing asecond signal proportional to said ullage temperature, pressuretransducer means providing a third signal proportional to said ullagepressure, and computer means for combining said first, second and thirdsignals to provide an output signal that is a function only of said fuelvolume.
 27. Apparatus as set forth in claim 26 in which said specificvolume detector comprises: a generally hollow housing of shieldingmaterial mounted in said ullage volume and having openings to permitsaid ullage to circulate therethrough, and a radiation sensor locatedinside said housing to receive radiation emitted only by the traceroccupying said housing.
 28. Apparatus for measuring the amount of fuelin a tank of known volume comprising: a source of radioactive traceremitting radiation, means for injecting a known amount of said tracerinto the ullage of said tank to establish a uniform tracer densitythroughout said ullage varying in accordance with the volume of saidullage, a detector for measuring the amount of radiation in a knownvolume of said ullage and generating a first signal proportional to saidfuel volume and the temperature and the pressure of said ullage,temperature transducer means providing a second signal proportional tosaid ullage temperature, pressure transducer means providing a thirdsignal proportional to said ullage pressure, a second detector forgenerating a fourth signal proportional to the amount of backgroundradiation in the environment of said fuel measurement, and computermeans for combining said signals to provide an output signalproportional only to said fuel volume.
 29. Apparatus for measuring theamount of fuel in a tank of known volume comprising: a frangible capsuleof compressed radioactive gas mounted in the ullage of said tank, meansfor rupturing said capsule dispersing a known number of curies of saidgas uniformly throughout said ullage, means for measuring the curiedensity of said ullage including a detector responsive to the radiationof said radioactive gas, a housing for enclosing a fixed sampling volumein said ullage and means for mounting said detector in said housing,said detector providing a first signal proportional to said fuel volume,pressure and the temperature of said ullage volume, and the externalradiation existing in the background of said curie density measurement,pressure transducer means providing a second signal proportional to saidullage pressure, temperature transducer providing a third signalproportional to said ullage temperature, a second detector forgenerating a fourth signal proportional to the intensity of saidbackground radiation, a first computer for combining said signals toprovide a first output signal proportional only to said fuel volume,second computer means receiving said output signal And said third signalfor providing a second output signal proportional to the mass of saidfuel in said tank, and means for registering an indication of saidsecond output signal.
 30. Apparatus as set forth in claim 29 whereinsaid frangible source comprises: at least one beta-emitting isotope. 31.Apparatus as set forth in claim 29 in which said sampling volume housingcomprises: a center cylindrical shield member having a centrally locatedbore for mounting said detector, a plurality of washers each having acentrally located bore, means for mounting said washers in verticalalignment on either side of said center cylindrical shield member and inalignment with the detector mounting bore thereof, means for spacingsaid washers slightly away from one another, and end plate means forsecuring said mounting means and defining said specific volume with saidaligned bores.
 32. Apparatus for determining the pressure variations ofa gas residing in a fixed interior volume at constant temperaturecomprising: means for injecting a radioactive gas into said residentgas, said tracer diffusing uniformly throughout said resident gas andproviding radiation having a range of penetration that is a function ofsaid resident gas pressure, and radiation detector means for providingan output signal proportional to said range of radiation penetration forindicating the pressure of said resident gas.
 33. Apparatus fordetermining the temperature of a gas residing in a fixed interior volumeat constant temperature comprising: means for injecting a radioactivegas into said resident gas said tracer diffusing uniformly throughoutsaid resident gas and providing radiation having a range of penetrationthat is a function of said resident gas temperature, and radiationdetector means providing an output signal proportional to said range ofradiation penetration for indicating the temperature of said residentgas.
 34. Apparatus for determining the volume of one fluid in thepresence of another immiscible fluid in a container, said apparatuscomprising: means for injecting into said container a known amount ofradioactive substance that preferentially diffuses uniformly throughoutonly said one fluid, and means coupled with said container for measuringthe density of said diffused radioactive substance to provide a signalindicative of the volume of said one fluid.
 35. The method as set forthin claim 11 but further including the steps of: introducing a secondtest gas of known quantity into said ullage volume, and measuring thecombined density of said first and said second test gases.
 36. In avolume measuring system, the combination of a container, a fluidoccupying a portion of the interior space in said container, meansconnected to said container for introducing a known mass of radioactivetrace gas into the ullage portion of said interior space and detectingmeans for the radioactive trace gas extending into said container formeasuring the density of said trace gas.
 37. A method of determining theullage volume of a partially filled container having a changeable volumeof material therein, comprising the steps of: introducing a radioactivegas into the volume of said vessel not occupied by said material thatdisperses throughout said non-occupied volume, measuring the density ofsaid gas in said non-occupied volume after it has diffused throughoutsaid volume, maintaining the mass of the radioactive gas substantiallyconstant while the determination is being performed and the materialvolume is subject to change, and correlating said measured density ofsaid radioactive gas with the ullage volume in said vessel.
 38. A methodof determining the ullage volume of a partially filled container havinga changeable volume of material therein, comprising the steps of:introducing a radioactive tracer gas into the volume of said vessel notoccupied by said mateRial that disperses throughout said non-occupiedvolume, measuring with a radioactive gas detector the density of saidgas in said non-occupied volume after it has diffused throughout saidvolume, maintaining the mass of the tracer gas substantially constantwhile the determination is being performed and the material volume issubject to change, and correlating said measured density of said tracergas with the ullage volume in said vessel.
 39. In a volume measuringsystem, the combination of a substantially closed container, means forproviding a mass of ullage gas to said container, means for introducinga known mass of radioactive trace gas into said ullage gas andradioactive gas detecting means extending into said ullage gas formeasuring the density of said trace gas.
 40. In a system for measuringliquid or gas quantity, the combination of a closed tank, containing aliquid or gas, whose quantity is to be measured, means for providing insaid closed tank a mass of ullage gas contiguous with said liquid orgas, means for introducing a known mass of radioactive trace gas intosaid ullage gas and radioactive gas detecting means extending into saidullage gas for measuring the density of said trace gas.
 41. In a liquidquantity measuring system in a G or zero gravitation environment thecombination of a closed tank containing a liquid whose quantity is to bemeasured, means for providing in said closed tank a mass of ullage gascontiguous with said liquid, means for introducing a known mass ofradioactive trace gas into said ullage gas and radioactive gas detectingmeans extending into said ullage gas for measuring the density of saidtrace gas.
 42. In a liquid quantity gauge for a zero gravitationalenvironment the combination of a closed tank having a diaphragmcontaining a liquid occupying a portion of the interior space in saidtank, means for providing a known mass of trace gas into the ullageportion of said interior space and detecting means extending into saidullage space for measuring the density of said gas.
 43. In a liquidquantity gauge for a zero gravitational environment the combination of aclosed tank having a diaphragm containing a liquid occupying a portionof the interior space of said tank, means for providing in said closedtank an ullage gas to the ullage portion of said interior space in saidtank, means for providing a known mass of trace gas into said ullagespace and detecting means extending into said ullage space for measuringthe density of said gas.